As one can ascertain, a pressure switch is a device that closes or opens an electrical contact when a pressure is above or below a certain preset threshold. These switches are utilized in a wide variety of applications, as in automobiles, aircrafts and in various other environments. By connecting the switch to a lamp or an indicator, the operator can discern if the pressure is normal or not. In this manner, a lamp will give a visual indication concerning the monitored pressure. Many pressure switches are electromechanical devices. Such devices typically include a Bourdon tube connected to the pressure source, which tube activates a snap action microswitch. Activation of the switch is associated with the operation of a light or other indicator showing that the pressure to be monitored has been exceeded. These switches are simple and relatively inexpensive, but have some serious disadvantages. For example, there is a poor accuracy of the operation point or the trip point of the switch, poor control of the hysteresis point of the switch. The switches also exhibit poor stability in regard to temperature and time and very often the switches provide poor reliability resulting in contact failures. Such failures can cause the contacts to fail in an open or closed position.
The prior art, of course, is cognizant of these disadvantages and mechanical switches are continuously being replaced by electronic solid state devices. For example, reference is made to U.S. Pat. No. 6,545,610 entitled, “Pressure Transducer and Switch Combination” which issued on Apr. 8, 2003 to A. D. Kurtz et al. and is assigned to Kulite Semiconductor Products, Inc., the assignee herein. In that patent, there is shown a device for providing a plurality of indications of a monitored pressure at a selected value. Basically, the device includes an electronic interface having an input and an output, a resistive structure adapted to measure the pressure and electrically coupled to the interface. There is a first output for providing a signal indicative of the selected pressure and the first output is electrically connected to the output of an electronic interface and a comparator coupled to the output of the electronic interface and adapted to provide a signal indicative of whether the monitored select pressure is greater than a predetermined threshold.
This is an example of an electronic pressure transducer and switch which includes a piezoresistive structure. The characteristic of the electronic switches are significantly better than the mechanical counterparts. This is due to the fact that reliability and stability is very good in regard to such switches and many systems use only electronic switches instead of the prior art mechanical devices. Such prior art mechanical devices are continuously being replaced by electrical switches. One problem is that the prior art mechanical devices are associated with a limited number of terminals, including an output terminal which receives a voltage and terminals associated with the lamp indicator. These are all pre-wired, as for example, in an aircraft or an automobile. Thus, besides replacing such a mechanical switch, one has to be sure that the contacts available for the mechanical switch, as the contacts available in the automobile or the aircraft, or any environment are totally compatible with the electronic switch as well. Accordingly, one should be able to directly replace the mechanical switch with the electronic switch without the utilization of any additional wiring. The purpose is to provide an electronic SPDT switch without making changes to the terminals in the switch operating environment, such as included in the automobile or the aircraft or some other device.